Water environment has undergone several state changes, and the factors responsible for causing these water state changes are different. In this paper, a fuzzy comprehensive-quantifying assessment (FCQA) is proposed to evaluate the water quality changes for a representative lake in central China in recent decades. The innovation assessment method develops an index, S mid , by applying a quantifying method to the fuzzy comprehensive assessment (PCA). The water quality of the lake is evaluated, and the temporal and spatial variations are analysed based on the data of 1991-2006. The Gray Filling Model [GM (1,1)] was used to predict the water quality trends in the subsequent 5 years, and the possible factors responsible for causing water environmental changes were analysed. The temporal and spatial variations of water quality show that there is a strong relationship between human activities and the water quality, and that excessive pen fish farming is the key internal pollution source threatening the lake at present.